original amount of banana chips = 10% x 15 lbs = 1.5 pounds
add x pounds of banana chips...... now you have 1.5 + x lbs of banana chips in the mix
the final mix has the same amaount of banana chips
the final mix is 25% banana chips the final weight is 15 + x
so the final amount of banana chips is .25(15+x)....
and as stated above, this equals 1.5 + x
sooooo 1.5 + x = .25(15+x) solve this for x......you will find x = 3 lbs